Optical sensor for sulfur dioxide determination in wines

A method for the determination of free and total sulfur dioxide in wines, based on the use of an optical sensor that employs a dichlorobis( diphenylphosphino) methane dipalladium I complex [Pd-2(dppm)(2)Cl-2] immobilized in a PVC membrane plasticized with o-nitrophenyloctylether (o-NPOE) is described. A sensing membrane [4.2% Pd-2(dppm)(2)Cl-2, 20.8% PVC, and 75% o-NPOE] was adapted to the tip of a bifurcated optical fiber bundle to perform reflectance measurements at 550 nm. The detection system consisted of two cells (40 mL), which hold the sample solution (plus reagents) and the optical sensor, respectively. For the determination of free SO2, a wine sample was mixed with H2SO4 solution in the sample cell, into which N-2 was bubbled, providing mixing of the solutions and conducting the SO2 formed toward the detection cell. For determination of total SO2, a KOH solution was mixed with the wine in the sample cell. Afterward, an H2SO4 solution was added to the cell, and then N-2 was bubbled to conclude the measurement. Linear responses up to 50 and 150 mg L-1 were obtained for free and total SO2, with detection limits of 0.37 and 0.70 mg L-1, respectively. The repeatability of the method was evaluated by carrying out 10 measurements using a single wine sample, providing relative standard deviation values of 2.2 and 2.5% for free and total SO2, respectively. The sensing membrane prepared from 10 mu L of the cocktail solution lasted for 80 measurements, whereas those prepared from 200 mu L can be used for 250 measurements. The method was applied to free and total SO2 determination in wines, and the results did not show significant difference from those obtained with the Ripper reference method at a confidence level of 95%.